The present invention relates to a volumetric phased array antenna system whose antenna elements are spatially arranged in three dimensions and which is often referred to as Crow""s Nest Antenna (CNA).
More concretely, the invention relates to a volumetric phased array antenna system comprising a number of antenna elements, each of which is connected to a T/R (transmitter/receiver) module, which is under the control of a beam steering computer (BSC), to which T/R module a transmitting signal is fed for forming a transmitting beam, and via which T/R modules RF signals are received and via a radar receiver are fed to a signal processing unit connected thereto. Such a volumetric phased array antenna system is known from H. Wilden, The crow""s nest radar-an omnidirectional phased array system, IEEE International Radar Conference, Arlington 1980, p. 253-258.
As is the case with all antenna systems, the CNA too is sensitive to interference sources, the signals from which are received in the side lobes of the antenna pattern. In military systems, interference signals are produced by the enemy to make intercommunication or target position measurements impossible. In civilian systems, such interfrence is caused by neighboring transmitting stations or by reflections from nearby objects.
When in the use of conventional radar antenna systems the location of the interference source is not known, use can be made, for the purpose of suppressing such interference or limiting the interference level, of so-called adaptive nulling systems, whereby one or more auxiliary antenna are arranged close to the main antenna. If one interference source is present, one auxiliary antenna is sufficient. The pattern of the main antenna is formed by a strong main lobe and a large number of weak side lobes; the antenna pattern of the auxiliary antenna is formed by a broad lobe which extends over at least the whole angular interval of the pattern of the main antenna, that is, over the entire field of view of the main antenna, but has a strength much smaller than that of the main lobe of the pattern of the main antenna. In the case of a sufficiently strong interference source, the interference signals received via the side lobes of the pattern of the main antenna may still be stronger than the reflection signals of the radiated radar beam received therein. Via the auxiliary antenna, practically always an interference signal will be received that is stronger than the signal coming from a target. It is known to extract from the target signals and interference signals received by the two antennas the target signal received by the main antenna in the main lobe, using algorithms developed therefor; for this purpose, a so-called nulling processor is used. It is further known, when there are several sources of interference, to also deploy several auxiliary antennas. To obtain maximum signal correlation and the highest possible interference suppression, it is important in these known systems that the auxiliary antennas are arranged close to the main antenna and that they all cover the same field of view of the main antenna. In planar and in linear phased array antennas, this is achieved by placing the auxiliary antennas in the same plane or in the same line as the main antenna. In a CNA this is not possible since there is not any plane containing all antenna elements. A possible solution for suppressing interference from an unknown interference source would be to arrange a large number of auxiliary antennas around the CNA. However, each auxiliary antenna requires its own receiver with pulse compression facility, Doppler processing, and so forth, so that the costs of such a solution become extremely high.
The object of the invention is to provide a design of a volumetric phased array antenna system, such that in a of relatively simple manner and at relatively low cost, an efficient suppression of interference can be realized in it.
To achieve this object, according to the invention, the volumetric phased array antenna system such as described in the preamble is characterized in that the antenna elements are arranged in mutually spaced conformal curved virtual surfaces having the same center of curvature or the same centers of curvature, with each combination of antenna elements in one or more surfaces together forming a volumetric phased array antenna or a part thereof.
Insofar as these surfaces have one and the same center of curvature, the virtual surfaces referred to form spherical shells or parts thereof. Thus, for instance, six of such spherical shells can be present, with each spherical shell potentially containing tens to hundreds of antenna elements. When these spherical shells are numbered 1 to 6 from the perimeter to the center, it holds, for instance, that the antenna elements in the outermost shell (shell 1) form an antenna for a weak and narrow beam, that the antenna elements in the innermost shell (shell 6) form an antenna for a weak and wide beam, that the antenna elements in, for instance, the outermost four shells (shells 1-4) form an antenna for a strong and narrow beam, and the antenna elements of the innermost four shells (shells 3-6) form an antenna for a strong and wide beam. It will be clear that all kinds of combinations of shells are possible. Thus, for instance, a main antenna can also be obtained by combining the antenna elements in the shells 1-5, and for the purpose of interference suppression an auxiliary antenna can be obtained by combining, for instance, the antenna elements in the shells 5 and 6. In all of these combinations, it is also possible, as in the planar array systems, to generate antenna patterns with several beams oriented in different directions.
The antenna elements located on the same virtual surface are connected via a T/R module to a single combination unit, while for an antenna pattern to be formed, a number of these combination units are connected to a further combination unit. If the antenna elements are to form, for instance, two antenna patterns where conventionally two discrete antennas would have to be used, two of such further combination units will be present. In this way, it is possible to form a fixed combination of antenna patterns, for instance a main antenna pattern and, for the purpose of interference suppression, two auxiliary antenna patterns. In such a situation, separate radar receivers for frequency down-conversion and detection of the radar signals will be connected to the further combination units, whereafter the thus detected signals can be further processed in a nulling processor. More difficult is the situation where the choice of the number of antenna patterns to be formed and the properties thereof has not been fixed. The further combination unit is then formed by a matrix switching unit for forming a number of antenna patterns that is to be set as desired, with beam properties that are to be set as desired. This measure therefore means that the discrete combination units are grouped as desired. This choice can naturally depend on, for instance, the extent of interference suppression in the nulling processor. A consequence of this setup, however, is that the discrete combination units must be connected directly to a radar receiver for frequency down-conversion and detecting the radar signals before these are fed to the matrix switching unit, which may render the costs of the entire radar system high again, after all. In practice, however, in, for instance, an interference suppression system with a main antenna pattern and one or two auxiliary antenna patterns, a fixed grouping of combination units will suffice.
Through the measures according to the invention, the following further advantages are obtained. Because the main antenna and auxiliary antennas are assembled into one integrated whole, this enables proper correlation of the signals obtained via these antennas, and hence proper interference suppression. The number of auxiliary antennas can be set as desired. The auxiliary antennas can be chosen so as to yield, to a considerable extent, the same antenna gain in all directions and hence equal interference suppression in virtually all directions. When the location of an interference source is known, the auxiliary antenna can be given an increased antenna gain in the direction of the interference source through steering by means of the beam steering computer, thus enabling further improved interference suppression.
In addition to being used for suppressing interference in military and civilian radar systems, the present invention can also be used for communication purposes.
When, for instance, in a communication system two users 10 at widely divergent distances are to be simultaneously served from the same communication station, then, through a different choice of combining shells of antenna elements, two beams in the direction of the respective users can be obtained simultaneously, such that the station is sufficiently sensitive to the distant user, but does not induce any saturation effects in the nearby user; in other words, the dynamic range of the receiving system of the station with an antenna construction according to the invention can be considerably limited.
When, in another example, in a communication system the service of a mobile user from a first station is taken over by a second station, then, after the takeover by the second station, it is possible in the first station, by means of a nulling system therein, to make the first station insensitive in the direction of the mobile user and hence in the direction of the second station.